Reduction in both size and power consumption of integrated circuits has led to the proliferation of low power sensors and wireless technology. For example, there are a wide variety of devices using low-power sensors, electronics, and wireless transmitters, separately or in combination, including tablets; smartphones; cell phones; laptop computers; MP3 players; telephony headsets; headphones; routers; gaming controllers; mobile internet adaptors; wireless sensors; tire pressure sensor monitors; wearable sensors that communicate with tablets, PCs, and/or smartphones; devices for monitoring livestock; medical devices; human body monitoring devices; toys; etc. Each of these devices requires a standalone power supply to operate. Typically, power supplies for these devices are electrical batteries, often replaceable batteries.
Other wireless technologies of significant interest are wireless sensors and wireless sensor networks. In such networks, wireless sensors are distributed throughout a particular environment to form an ad hoc network that relays measurement data to a central hub. Particular environments include, for example, an automobile, an aircraft, a factory, or a building. A wireless sensor network may include several to tens of thousands of wireless sensor “nodes” that operate using multi-hop transmissions over distances. Each wireless node will generally include a sensor, wireless electronics, and a power source. These wireless sensor networks can be used to create an intelligent environment responding to environmental conditions.
A wireless sensor node, like the other wireless devices mentioned above, requires standalone electrical power to operate the electronics of that node. Conventional batteries, such as lithium-ion batteries, zinc-air batteries, lithium batteries, alkaline batteries, nickel-metal-hydride batteries, and nickel-cadmium batteries, could be used. However, it may be advantageous for wireless sensor nodes to function beyond the typical lifetime of such batteries. In addition, battery replacement can be burdensome, particularly in larger networks with many nodes.
Alternative standalone power supplies rely on scavenging (or “harvesting”) energy from the ambient environment. For example, if a power-driven device is exposed to sufficient light, a suitable alternative standalone power supply may include photoelectric or solar cells. Alternatively, if the power-driven device is exposed to sufficient air movement, a suitable alternative standalone power supply may include a turbine or micro-turbine for harvesting power from the moving air. Other alternative standalone power supplies could also be based on temperature fluctuations, pressure fluctuations, or other environmental influences.
Some environments do not include sufficient amounts of light, air movement, temperature fluctuation, and/or pressure variation to power particular devices. Under such environments, the device may nevertheless be subjected to fairly predictable and/or constant vibrations, e.g., emanating from a structural support, which can be in the form of either a vibration at a constant frequency, or an impulse vibration containing a multitude of frequencies. In such cases, a scavenger (or harvester) that essentially converts movement (e.g., vibrational energy) into electrical energy can be used.
One particular type of vibrational energy harvester utilizes resonant beams that incorporate a piezoelectric material that generates electrical charge when strained during resonance of the beams caused by ambient vibrations (driving forces).
Microelectromechanical (“MEMS”) piezoelectric energy harvesters with silicon cantilevers typically have a cross-section consisting at least of oxide/cantilever material/piezoelectric stack/oxide (the oxide is typically deposited silicon dioxide). The silicon material used for the cantilever is typically formed from the single crystalline silicon device layer of a silicon-on-insulator (“SOI”) wafer. A second piezoelectric stack is often placed in the device structure to form a dual piezoelectric stack in order to increase the power output from the energy harvester. The additional piezoelectric stack is placed on the same side of the cantilever as the first piezoelectric stack (or on top of the first piezoelectric stack), in the format of at least oxide/cantilever material/piezoelectric stack/oxide/piezoelectric stack/oxide. The additional piezoelectric stack is placed in this manner because during the process used to fabricate SOI wafers, direct bonding of two silicon wafers at high temperature, followed by grinding and polishing steps, makes it extremely difficult to place piezoelectric stacks in between the device and handle wafers.
Placement of the additional piezoelectric stack on the same side of the cantilever as the first piezoelectric stack creates a lack of symmetry in cross-section with respect to the piezoelectric stack, requiring the piezoelectric stack residual stress to be tuned in order to engineer the flatness of the cantilever. Curl or lack of flatness in the cantilever due to poor management of residual stress in the layers can impact the performance of the MEMS energy harvester. Tuning the residual stress of the piezoelectric stack can also impact its inherent piezoelectric properties and, thus, device performance. A tradeoff must, therefore, be made in the stresses required for cantilever flatness and for quality piezoelectric response.
The present invention is directed to overcoming these and other deficiencies in the art.